The pathogenesis and natural history of herpesvirus infections in man are being investigated. Patients are being identified, followed clinically and diagnosed definitively by virus isolation. Studies of patients with infectious mononucleosis have documented a variety of effects of the causative agent, the EB virus, upon the target B lymphocyte. The development and nature of T-cell mediated immune responses to the EBV infected B cells have been characterized. The results of initial trials of adenine arabinoside therapy of serious herpesvirus infections have been substantiated. The tools are being developed to study the biology and molecular epidemiology of varicella-zoster virus infection.